I recently started having the problem that my Apache crashed and could not be restarted. The hosting company then told me that it has to do with 'semaphores' and sent me this snipped with which they solved the problem:

/usr/bin/ipcrm sem $(/usr/bin/ipcs -s | grep www-data | awk '{print$2}')

Now that's nice to have a command to execute that solves my problem but then again I have no clue what this is all about.

What are semaphores and who the heck puts them, where they are and how do they crash my apache?

I'd be really glad for some general explanations!